Variable reactance circuit



March 13, 1934. F. E. TERMAN VARIABLE REACTANCE CIRCUIT Filed May 25,1931 INVENTOR, FREDERICK E. TERM/W ATTORNEY Patented Mar. 13, 1934UNITED STATES PATENT OFFICE This invention relates to variablereactanccs, and particularly to reactances which are cyclically variableat high frequency, and which may be utilized as modulators to produceside- 5 band frequencies without carrier frequencies, in accordance withthe method set forth in my application for United States LettersPatenton a Frequency changing system, filed October 20, 1930, Serial No.489,917.

Among the objects of my invention are: first, to provide a circuithaving an effective reactance which may be cyclically varied at highfrequency; second, to provide a circuit adaptable for use in a frequencychanging system of the character-set forth in my above identifiedapplication;' third, to provide a circuit having a reactancecorresponding either to an inductance or a capacitance in its variationwith frequency,

tive phase rotations of the two branches are balanced over widefrequency bands.

Other objects and advantages of my invention will become apparent, orwill be specifically pointed out, in the course of this specification.

I have pointed out in my above mentioned application, Serial No. 489,917that if a modulating wave, such as the electrical equivalent of a soundwave, be passed through an impedance which is cyclically varied at acarrier frequency, there will be present in the circuit currents offrequencies corresponding to the sum and the difference of the carrierfrequency and the modulating frequency, but without the presence of thecarrier frequency current itself. In that apfrequency, i. e., whereinthe positive and nega-- plication, and in my co-pending application,Se-' The of the tube there is inserted a variable resistance element,such, for example, as one of the cyclically variable vacuum tuberesistor arrangements described in my above mentioned application,Serial No. 539,655. This resistive 90 element may, if desired, comprisea dynatron tube having'a negative resistance. If the reactance of thefirst mentioned tube be measured between its cathode and grid terminals,it will be found to have a frequency characteristic of the samecharacter as the coupling between grid and anode, i. e., if the couplingbe capacitive, the reactance measured between cathode and grid willdecrease with increasing frequency. The magnitude of this reactance willvary inversely with the resistance of the element in its output circuit,and will have the same sign as that resistance. That is to say, if theresistance in the output circuit be negative, as with a dynatronelement, the reactance between grid and cathode will be of the same signas an inductance.

Fig, 1 is a simple form of circuit in accordance with my invention.

Fig. 2 is a preferred form of a cyclically variable reactance circuit.

Fig. 3 is a modification of my invention,

. which may be utilized to produce an effective negative variablecapacitance.

One of the simplest formsof such a circuit is illustrated in detail inFigure l. The terminals 1 and 2 of the variable-reactance circuit areconnected respectively to the grid 3 and cathode 4 of a triode 5 ofordinary construction. The plate 6 of the tube is coupled to the grid 3through a capacitance 7, which may be the internal capacitance of thetube, but is preferably an external capacitance connected around thetube. Anode current for the tube is supplied from a source 8 which feedsthe plate through a choke coil 10 having an impedance which is high ascompared with the tube resistance at all frequencies for which thecircuit is designed.

Connected across the choke 10 is a variable resistor 11, in series withwhich a blocking condenser 12 is'preferably connected, this resistancethus being the effective load circuit of the tube and connected betweenthe anode and cathode.

6. The effective input capacitance of the circuit as a whole under thesecircumstances is equal to the capacitance C; between cathode and grid,plus (1+u')C2, and where C2 is the capacitance of the condenser 7 and uis the effective amplification of the tube. This eflective amplificationu is equal to u, the amplification constant of the tube, times R/(R+r),where R is the resistance of the element 11, and r is the internalresistance of the tube.

It will therefore be seen that as the resistance R of the element 11 isvaried, the eflective capacitance of the input circuit will vary in thesame manner, or in other words, the effective reactance of the circuitwill vary inversely with A preferred method of applying this principleto produce a cyclically varying reactance, is shown in Figure 2. Theinput terminals 1' and 2' are connected respectively to the grid 3' andcathode 4 of the tube 5' as before. In this case the coupling betweenanode and grid comprises an inductor 15 in series with a blockingcondenser 16, connected across grid and anode terminals as before, inorder to produce an inductive reactance in the circuit as a whole. Theplate of the tube is supplied with current from the source 8' throughthe choke 10'.

The load circuit in this case, however, comprises a tetrode 17 whosefilament 18 connects with the filament of the tube 5. A choke 19.connects from the filament to the plate 21 to hold the latter at zerobias, the plate 21 also connecting with the plate 6' through a blockingcondenser 20. The shield grid 22 may derive its positive bias from a tapon the source 8', while the control grid 25 is supplied from a source 26of carrier frequency potential.

As disclosed in my copending application, Serial No. 539,655 abovementioned, the tetrode 17, connected in this manner, becomes a resistorwhose resistance varies cyclically with the potential of the carrierfrequency source. The reactance across the terminals 1' and 2' willtherefore be an inductive reactance varying in the same manner as thepotential of the carrier frequency source 26.

Other types of resistive elements, either varying cyclically, or ofconstant value, may be utilized in like manner to give varying eflect.Thus there is shown in Figure 3 a circuit which is operative to give aneffective negative variable capacitance across the terminals 1" and 2".The tube 5" whose elements are similarly numbered to those of the tube 5in Figure 1,

but are distinguished by double accents, has its grid and anodeconnected by a capacitance '7" as in the first figure.

In this case, however, the resistive output circuit comprises a dynatron30, the latter being a tetrode whose cathode 31 and control grid 32 aresupplied from a carrier frequency potential source 33. The plate 35connects directly to the anode6" of the triode. The screen grid 36connects to the filament circuit of both tubes through a source 37 whichmaintains a screen grid of the highest potential in the circuit. A tap38 on the battery or other source 3'7 supplies a somewhat lowerpotential through the choke coil 40 to the anodes of both tubes.

As is well known, if the potentials of the anode 35 and the screen grid36 be properly adjusted, secondary emission of electrons from the anodewill cause the net current flowing in the anode circuit to decrease asthe voltage rises, the tube therefore acting as a negative resistance.By varying the potential of the grid 32 by means of a carrier frequencysource 33, the magnitude of this negative resistance may be varied in acyclical manner. Under these circumstances a negative capacitance willbe displayed across the terminals 1" and 2", or, if

an inductance be substituted for the capacity 7" in the same manner asthe inductance 15 is connected in Figure 2, the circuit will display anegative inductance.

The meaning of these negative reactive quantities may be stated asfollows: The magnitude of the current flowing past the terminals 1" and2" varies in the same manner as in a positive reactance. That is, if thecircuit be acting as a negative capacitance, the reactance offered willdecrease with increasing frequency. The phase will, however, be reversedas compared to that of the current in a true capacitance, the voltageleading the current instead of the current leading the voltage. If thenegatively reactive circuit be inductive, the reactance will increasewith increasing frequency, but the current flowing will lead the voltageinstead of lagging as in a positive inductive circuit.

The use of reactive elements of opposite phase characteristics tobalance each others effect is well known in resonant circuits. In such acircult the reactances, at the frequency of resonarice, are equal but ofopposite phase. By using negative reactances of the character abovedescribed, such resonant circuits may be made in which the reactiveeffects are balanced out at all frequencies over which the device isoperated. That is, since the positive and negative reactive elementshave similar frequency characteristics, a circuit comprising a positiveand negative reactance will be in balance at all frequencies if it be inbalance at any particular frequency. Resonance, in the sense of zerophase rotation or unity power factor, may therefore be obtained overextremely wide frequency bands.

It the negative reactance be made variable, by varying the negativeresistance inthe output circuit of the tube, it is thus possible toutilize a combined circuit, in which positive and negative reactancesare balanced, as a modulating circuit in the same manner as a single,unbalanced, reactive circuit is used.

I claim: v

1. The method of operating a triode as a variable impedance element in acircuit including a reactive coupling between the grid and anodeelements of said triode, and a resistance connected between the cathodeand anode thereof, which comprises varying said resistance to produceconsonant variations of the effective anode-grid reactance of saidtriode.

2. The method of operating 'a triode as a variable impedance element ina circuit including a reactive coupling between the grid and anodeelements of said triode, and a resistance connected between the cathodeand anode thereof, which comprises cyclically varying said resistance toproduce consonant variations of the effective anode-grid reactance ofsaid triode.

3. A circuit of variable reactance comprising a vacuum tube including acathode, an anode, and a grid and having reactive coupling between saidanode and grid, and a second vacuum tube connected as an output resistorbetween the anode and cathode of said first tube, said second tube beingvariable in resistance to produce consonant variations in reactancebetween the cathode and grid of said first tube.

4. A circuit of variable reactance comprising a vacuum tube including acathode, an anode, and a grid, a reactive element connected between saidgrid and anode, and a second vacuum tube connected as an output resistorbetween the anode and cathode of said first tube, said second tube beingvariable in resistance to produce consonant variations in reactancebetween-the cathode and grid of said first tube.

5. A circuit of variable reactance comprising a vacuum tube including acathode, an anode, and a grid, a capacitance externally connectedbetween said grid and anode, and a second vacuum tube connected as anoutput resistor between the anode and cathode of said first tube, saidsecond tube being variable in resistance to produce consonant variationsin reactance between the cathode and grid of said first tube.

6. A circuit of variable reactance comprising a vacuum tube including acathode, an anode, and a grid, an inductance connected between saidcathode and anode, and a second vacuum tube connected as an outputresistor between the anode and cathode of said first tube, said 'secondtube being variable in resistance to produce consonant variations inreactance between the cathode and grid of said first tube.

'7. A circuit of variable reactance comprising a vacuum tube including acathode, an anode, and a grid and having reactive coupling between saidanode and grid, and a second vacuum tube having a negative resistivecharacteristic connected as an output resistance between the anode andcathode of said first tube, whereby said first tube displays an inputreactance bay-- ing frequency characteristics corresponding to thereactive coupling between its cathode and anode and of opposite phasecharacteristics.

8. A circuit of variable reactance comprising a vacuum tube including acathode, an anode, and a grid and having reactive coupling between saidanode and grid, a second vacuum tube having a negative resistivecharacteristic connected as an output resistance between the anode andcathode of said first tube, and means for varying the efiective negativeresistance of said second tube.

9. A variable reactance circuit comprising a vacuum tube including acathode, an anode, and a grid, and having reactive coupling between saidanode and grid, a second vacuum tube connected as an output resistorbetween said anode and cathode, and means for cyclically varying theefiective resistance of said second tube to produce correspondingvariations in the efiective cathode-grid reactance of said first tube.

10. The method of employing an amplifier tube having input and outputcircuits to'vary the impedance of said input circuit which com-

